The present invention relates to semiconductor fabrication. More particularly, the present invention relates to cleaning of semiconductor devices by applying sonic vibrations to remove impurities or foreign material from the devices.
Semiconductor devices are employed in various systems for a wide range of applications. These devices are fabricated in a series of processing steps. The steps may include depositing material on a semiconductor wafer, patterning the material, etching selected portions of the material, doping, cleaning the semiconductor wafer and repeating one or more of these steps. Typically, up to one fifth of all processing steps involve some form of cleaning. As used herein, the term “semiconductor wafer” includes any substrate, microelectronic device, chip or the like, at any stage of fabrication, which is used to form an integrated circuit or other electronic circuitry.
Cleaning removes unwanted particles from the semiconductor wafer. As used herein, a “particle” means any impurity, foreign particle or other material that is unwanted or is not supposed to be present on a surface of the semiconductor wafer. For example, particles include organic and inorganic residues introduced by prior wafer processing steps. If not removed, particles may adversely affect device fabrication or performance. The direct impact of these particles is a deterioration of manufacturing chip yield. For example, particles may interfere with subsequent deposition or etching steps by covering areas of interest. Particles may cause short circuits by blocking subsequent dielectric depositions between conducting lines, e.g., when the conducting lines are connected by foreign particles that are conductive.
The cleaning process typically involves applying a cleaning solution to the surface of the semiconductor wafer. There are various cleaning solutions that are used. By way of example, one such solution is called the “standard clean 1” (SC1), which includes alkaline solutions of, e.g., hydrogen peroxide (H2O2) and ammonium hydroxide (NH4OH) in deionized water. The “Huang A” cleaning solution includes the same chemistry. Another cleaning solution is called the “standard clean 2” (SC2), which includes, e.g., H2O2 and hydrogen chloride (HCl) in deionized water. A conventional “RCA” cleaning process employs an SC1 sequence followed by an SC2 sequence. These solutions are prepared by mixing the chemical components with water to achieve the desired ratio. These and other cleaning solutions (and sequences) are also known as aqueous solutions.
Sonic waves (i.e., sound waves) may be applied to the cleaning solution in order to enhance the cleaning process. Sonic waves are typically produced by a transducer external to a wafer-cleaning tank. Ultrasonic waves on the order of tens to hundreds of kilohertz (KHz) or megasonic waves on the order of millions of hertz (MHz) are typically used. Megasonic and ultrasonic waves produce acoustic cavitation and rectified diffusion in the cleaning solution. Cavitation is the sudden formation and collapse of low-pressure bubbles. In rectified diffusion, bubbles will oscillate due to an (externally) applied acoustic energy source. There is diffusion of gas into these bubbles during one-half cycle of the acoustic wave. During the other half-cycle of the acoustic wave, there is diffusion out of the gas bubbles. The diffusion rates are not similar and therefore the bubbles increase in size and oscillate. When the bubbles created by the cavitation collapse, energy is imparted to the particles. The energy is typically sufficient to dislodge or “scrub” the particles from surfaces of the semiconductor wafer. One drawback to using ultrasonic waves is that at lower frequencies, for example in the range of 10 KHz to 70 KHz, the energy released when the bubbles collapse may be great enough to damage the semiconductor device. It has been hypothesized that even at higher frequencies, significant damage may occur in small-scale devices.
The cavitation process includes nucleation, bubble growth and collapse of the gas bubbles in an applied acoustic energy source. Homogeneous nucleation occurs when the cleaning solution is free of impurities or additives and the reaction vessel is defect-free. Particles on a wafer could act as a nucleating source. Defect sites can be, e.g., small cracks on the walls of the vessel or chamber in which cavitation occurs. Homogeneous nucleation is difficult to control, because the absence of any defect sites means that the gas bubbles have to form within a liquid phase. It is difficult to repeat or controllably produce the same number of homogenous nucleating bubbles in subsequent semiconductor batches. Because particle removal is understood to take place through the transfer of energy from the oscillating bubbles, if the original number of bubbles is different from run to run among different semiconductor batches, there would be differences in particle removal efficiency. This, in turn, means that the cavitation process is not as effective as it could be. Thus, particle removal efficiency suffers because of the lack of control.
Another drawback to current cleaning processes is inefficient removal of small-size particles. Improvements in semiconductor device manufacturing, such as decreased cost and increased capacity, are often achieved by shrinking device size. As devices shrink, small particles become more problematic. By way of example only, the feature size of a semiconductor device may be on the order of a fraction of a micron (e.g., 0.1 to 0.5 μm). Small particles, for example those on the order of 0.15 μm or less, may be held to a surface by forces such as the van der Waals force or by capillary force. Unfortunately, conventional ultrasonic and megasonic cleaning techniques may not remove a necessary amount of such small particles as effectively as they remove larger size particles. Therefore, a need exists for more robust nucleation processes that improve particle removal.